The concentrations of bicarbonate (HCO3-) and carbonate (CO32-) in your irrigation water should be evaluated carefully. Applying water that's high in bicarbonate can increase the pH of the soil. Applying water containing high levels of both bicarbonate and carbonate can decrease the permeability of the receiving soil — an undesirable outcome. Finally, sprinkling can be a problem. If bicarbonate is high and the irrigated water is sprinkled on plants during hot, dry weather, white lime deposits (solid CaCO3) may form on the plant's leaves as a result of evaporation.
The bicarbonate hazard of water may be expressed as residual sodium carbonate (RSC), calculated as follows:
RSC = ( HCO3- + CO32- ) — ( Ca2+ + Mg2+ )
where concentrations of ions are expressed in milliequivalents per liter.
Under most circumstances, water that has an RSC value of lower than 1.25 can be used safely for irrigation, whereas water with an RSC of 1.25 to 2.5 is marginal. Water that has an RSC of 2.5 or more typically is not suitable for irrigation and should be avoided.
Note that recycled waters typically contain higher concentrations of bicarbonates and carbonates than do freshwater sources. That's because during the water recycling process any ammonia nitrogen (NH4-N) in the water is oxidized to nitrate nitrogen (NO3-N), producing acidity that must be neutralized by subsequently adding a base — either lime (CaCO3) or soda ash (Na2CO3). Both of the latter are salt compounds that contribute carbonates and bicarbonates to the water upon dissolving.
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